Sediments are generally layered. Again and again more or less horizontal layers are formed on top of older layers. The layers generally contain sedimentary structures, not to be confused with the layering.

In folded layers, the original stratification is often difficult to determine. Due to the pressure of the folding the cleavage direction of the rock can be changed. The cleavage is no longer parallel to the layering. In these cases, it helps to examine the rocks in the detail to find differences in structure, grain size and type of sediment.

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